1. Field of the Invention
The invention relates to devices that can be used to eliminate released or accidentally formed hydrogen from non-inerted spaces, e.g., safety containers of pressurized water reactors and non-inerted boiling water reactors, which contain steam, air, aerosols and other gases in addition to hydrogen, effectively without backfiring. In this case, the hydrogen can be recombined into steam within the device in the presence of the existing atmospheric oxygen, e.g., in a catalytic procedure.
2. Background Information
During the course of serious accidents, large amounts of hydrogen are formed in light-water reactors (LWR) due to the reduction of steam, which get into the safety containers. The maximal hydrogen amounts in both pressurized and boiling water reactors can measure about 20,000 mn3. There is also the danger that the atmospheric air in the safety containers (containments) will give rise to flammable mixtures, whose uncontrolled ignition and subsequent detonation places a serious dynamic compressive stress on the containment walls. In addition, steam and hydrogen always lead to pressure and temperature increases in the accident atmosphere. This is particularly significant in boiling water reactors, since their container volumes measure only about 20,000 mn3, in comparison to 70,000 mn3 in pressurized water reactors. Pressure and temperature increases result in an additional static stress on the containment walls. Further, leaks owing to excess pressure can give rise to the emission of radiotoxic substances.
Precautionary safety measures involve inerting the gas volumes with nitrogen, as has already been done for boiling water reactors. Catalytic recombinators represent countermeasures that have been discussed and partially implemented already. These are used to exothermally catalytically recombine the formed hydrogen both inside and outside the limits of inflammability, i.e., convert it into steam with the generation of heat. Hydrogen contents with concentrations lying within the limits of inflammability can also be burned off in a conventional manner after spark ignition. However, the resultant processes are not controllable, so that the system-jeopardizing reactions already mentioned above can arise under certain conditions.
In order to eliminate the hydrogen arising during normal operation and as the result of an accident, both thermal and catalytic recombinators were developed, which recombine the hydrogen with the oxygen in the air to form steam. Preference is given to catalytic systems, which operate passively, i.e., are self-starting and need no external power supply, so as to ensure availability during an accident. Substrates used in the known recombinators include metal plates or films as well as highly porous granulate, on which platinum or palladium is applied as the catalyst. Several films and granulate packets (the granulate is held together in packets by wire mesh) are arranged vertically and parallel to each other in sheet casings. The hydrogen/air mixture enters into the casing from below. The reaction starts on the catalytically coated surfaces. The mixture or reaction products stream over the substrate surfaces.
To date, the recombinators have made use of bilaterally coated plates or films and granulate packets. Their surfaces are homogenous, i.e., covered with constant amounts of precious metal. In addition, all catalyst elements are completely coated.
As a result, the dissipation of reaction heat from the systems is basically problematical. It is accomplished almost exclusively via convection from the solid surfaces on the gases streaming past, and heat radiation to neighboring structures. However, excessive hydrogen amounts can cause the coated substrates to become overheated, so that the ignition temperature is reached or exceeded, so that homogenous gas-phase reactions with deflagration or detonation can come about. One other disadvantage lies in the additional heating of the immediate environment of the substrates.